Typically, "slower" updates in Debian testing are due to the fact that the new package version has known breakage that makes it unsuitable for release, or requires dependencies to be updated and this in turn breaks existing packages (either the build, or the package itself). These situations have to be managed via a "transitions" mechanism. When in doubt, there is a tracker at https://qa.debian.org/excuses.php that reports on why a newer version of the package has yet to enter the testing channel.